This invention relates to a power-supply arrangement comprising
a series arrangement of at least a primary winding of a transformer, a transistor switch and a first resistor connected between a first and a second input terminal for receiving an input voltage, PA1 the series arrangement of at least a secondary winding of the transformer and a diode connected between a first and a second output teminal for connecting a load, PA1 a pulse-width modulator for supplying control pulses to the transistor switch, PA1 an oscillator for supplying a sawtooth signal to a first input of the pulse-width modulator, and PA1 a control amplifier for supplying to a second input of the pulse-width modulator a first feedback signal which is a measure of the difference between a reference signal and a signal which is proportional to an output signal of the power-supply arrangement. PA1 the oscillator is adapted to supply a negative-ramp sawtooth signal, PA1 the end of the first resistor which is connected to the second input terminal is connected to a point at ground potential, and PA1 the second terminal of the capacitor is coupled to that end of the first resistor which is not connected to the second input terminal. PA1 the oscillator is adapted to supply a positive-ramp sawtooth signal, PA1 the end of the first resistor which is not connected to the second input terminal is connected to a point at ground potential, and PA1 the second terminal of the capacitor is coupled to that end of the first resistor which is connected to the second input terminal. PA1 a first control amplifier for supplying a feedback signal which is a measure of the difference between a reference signal and a signal which is proportional to the average charging current of the battery and which appears on the second input of the pulse width modulator if the load is constituted by the battery, and PA1 a second control amplifier for supplying a feed-back signal which is a measure of the difference between a reference signal and a signal which is proportional to the motor voltage and which appears on the second input of the pulse-width modulator if the load is constituted by the battery in parallel with the motor.
Such a power-supply arrangement is suitable for general purposes and is particularly suitable for use in a rechargeable shaver for charging the batteries and/or powering the motor.
Such a power-supply arrangement is known from the article "Steuer- und Regelschaltung TDA 1060 fur Schaltnetzteile" in Technische Information fur die Industrie, April 1977. In this known arrangement the output voltage of the arrangement is maintained substantially constant in that the difference between a reference voltage and a fraction of the output voltage is amplified and is fed back to a pulse-width modulator which compares this voltage with a sawtooth voltage to control the width of the control pulses for the transistor switch.
The output voltage of the power-supply arrangement depends on the input voltage. For a constant pulse width, when the input voltage increases, the maximum value of the current through the primary winding increases, so that the energy stored in the transformer also increases. In order to minimise variations of the output voltage as a result of input-voltage variations the arrangement employs a feed-forward control, i.e. a voltage proportional to the input voltage is applied to the oscillator in such a way that the rise time of the sawtooth voltage decreases as the input voltage increases. When the input voltage increases this results in a decreasing width of the control pulses for controlling the transistor switch. However, this control method has the drawback that allowance is only made for variations of the input voltage, which variations directly influence the width of the control pulses. Therefore, the output-voltage variations are still comparatively large so that the control range of the feedback loop must be comparatively large and, consequently, the components used therein must have a comparatively large bandwidth. Moreover, these comparatively large variations complicate stabilisation of the feedback loop. Therefore, it is an object of the invention to provide a power-supply arrangement whose output-signal variations are comparatively small.